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In the past, people had insufficient understanding of heat transfer oils; they believed that as long as such oils were kept at a certain temperature, they could be used for a long time without coking, or they thought that if the oils deteriorated over time, it was sufficient to replace them, and then coking would not occur on the furnace tubes. Whether heat transfer oil will clog the walls of the furnace tubes, and why its performance in heat transfer deteriorates after being used for a certain period of time, are topics that have been the focus of discussion in the heat transfer oil industry in recent years. In fact, like other natural substances, heat transfer oil can also go through a process of deteriorating in quality. When used in hot oil furnaces, heat transfer oil develops scaling, just as water droplets form scale on the surface of a pond; this scaling occurs in four main stages. First, during the induction stage, under the heating effect on the metal surface of the furnace tubes, two main types of chemical reactions take place: one is cracking reaction, and the other is condensation reaction. The cracking reaction breaks down large alkane molecules into smaller ones, and the physical properties of heat transfer oil change as viscosity decreases and the flash point drops ； The condensation reaction causes alkane macromolecules to condense into larger molecules such as polycyclic aromatics or fused ring aromatics, resulting in an increase in the viscosity and flash point of the heat transfer oil. Since the hot oil furnace has an expansion tank, and this expansion tank is in contact with air, a portion of the heat transfer oil with a low flash point evaporates ； Furthermore, during the induction phase, the main chemical reaction of the heat transfer oil is a condensation reaction; therefore, when the heat transfer oil becomes unusable, its physical properties such as viscosity, flash point, acid value, and residue carbon content all increase. In these chemical reactions, the main reaction pathway is: alkanes -> alkenes -> aromatic hydrocarbons -> polycyclic aromatic hydrocarbons -> resins -> asphaltenes. It can be seen that, in this reaction process, the relative molecular mass gradually increases. For example, the relative molecular mass of gelatinous substances ranges from 600 to 1000, while that of asphaltenes ranges from 700 to 40,000. These macromolecular substances are insoluble in the heat transfer oil and are separated from it. The separated gums and asphaltenes are viscous in nature; they act as catalysts in heat transfer oils, continuing to induce the heat transfer oils to undergo dehydrogenation reactions through heating and condensation. II. During the adsorption stage, the heat transfer oil is heated to produce asphaltenes, which then migrate toward the metal surface of the furnace tube or are adsorbed by that surface. Adsorption is a surface phenomenon in which the asphaltenes in the heat transfer oil migrate across the metal surface of the furnace tube. Adsorption can be divided into physical adsorption and chemical adsorption. Physical adsorption generally occurs at lower temperatures and is the result of van der Waals forces; no electron pairs are formed. It can be either single-molecule adsorption or multi-molecule adsorption, and it is not necessary for a second layer to be adsorbed only after the first layer is fully occupied ； It’s not necessarily the case that the third layer is adsorbed only after the second layer is fully saturated; it’s an irregular adsorption process. Chemical adsorption, on the other hand, can only occur in a monolayer fashion, and compounds are formed during the adsorption process. In heat transfer oil, the asphaltenes adhere to the metal surface of the furnace tubes primarily through physical adsorption, and the thickness of this adsorption layer is uneven. When the temperature increases, carbon and steel may undergo chemical adsorption to form compounds, which can make the furnace tubes brittle and affect their quality. III. In the hardening stage, asphaltene adheres to the walls of the furnace tube; further heating causes it to harden and form coking. Coking is relatively hard, has a very low heat transfer coefficient, and is a non-heat-transferring material. Adding a coking layer to the metal surface provides insulation, so coking is not beneficial in the use of heat transfer oil furnaces. During the hardening stage, the main chemical reaction involved in coking is dehydration. Depending on the degree of dehydration, different shapes of coke are formed, which can be categorized into the following three types: ① Sponge coke, also known as amorphous coke, has a low C/H ratio; it is loose in structure, soft in hardness, contains a high amount of oil, burns easily in a flame, continues to burn even after leaving the flame, and is easy to remove from the furnace tubes. Spongy coke is formed from oils with a low aromatic content. ②Honeycomb coke, also known as isomorphous coke, has a moderate C/H ratio. Its internal structure is honeycomb-shaped, it has moderate hardness, contains little oil, does not burn easily in a flame, remains unburned when away from the flame, and is difficult to remove from furnace tubes. Coccoal is produced from oils with a moderate aromatic content. ③Pin coke, also known as heteroform coke, has a high C/H ratio. The pores within the coke mass are uniformly oriented and appear as elongated oval shapes. When broken, the coke mass splits into pin-shaped fragments or glass-like pieces; it is highly hard, contains very little oil, produces sparks when exposed to flame, and behaves like a stone once removed from the flame. It is difficult to remove this type of coke from furnace tubes. Pinacol coke is produced from oils with a high aromatic content. The performance comparison of various heat transfer oils in terms of coking is shown in the table below. Coke type, structure, C/H ratio, appearance, hardness, method of formation, difficulty in cleaning. Sponge-shaped coke: amorphous, low porosity, low aromatic content; oil is easily produced. Honeycomb-shaped coke: isotropic, honeycomb structure, moderate aromatic content; oil is difficult to produce. Needle-shaped coke: high anisotropy, uniform pores, high aromatic content; oil is very difficult to produce